A flexible wiring board includes an insulative polyimide resin layer that is directly deposited over conductors, such as copper foil, without the help of any adhesive layer. Such a flexible wiring board is generally manufactured by the following process. First, acid dianhydride and diamine are polymerized in a solvent such as N-methyl-2-pyrrolidone to obtain a polyimide precursor containing a polyamic acid. The polyimide precursor is then applied over a conductor and is dried to form a layer containing a polyamic acid. This layer is heated to imidize the polyamic acid and thus form a polyimide resin layer over the conductor.
One problem with this approach is that the conductor such as a copper foil has a relatively low coefficient of linear thermal expansion while polyimide generally has a relatively high coefficient of linear thermal expansion, so that the difference in coefficient of linear thermal expansion between the conductor and the polyimide causes the flexible wiring board to curl. The flexible wiring boards also tend to curl as the polyimide layer undergoes dehydration and contracts during formation of polyimide and internal stress builds up between the resulting polyimide and the conductor.
The curling of flexible wiring boards can be reduced by using a polyimide with a relatively low coefficient of linear thermal expansion to minimize the difference in coefficient of linear thermal expansion between the conductor and the polyimide. To decrease the coefficient of linear thermal expansion of polyimides, highly linear monomers such as pyromellitic dianhydride, 3,4,3′,4′-biphenyltetracarboxylic dianhydride and p-phenylenediamine are used in combination to form a polymer. The internal stress that builds up during the formation of polyimide can be removed by heating the imidized polyimide film above the glass transition temperature to plasticize the polymer.
When a polyimide precursor applied over conductors is thermally imidized, the imidization process must be carried out at significantly high temperatures to ensure that the precursor is completely imidized (100% imidization). Moreover, the polyimide film must be heated to a temperature of at least 300° C. or above, typically 350° C. or above, to remove internal stress. Such high temperature heat treatment may result in oxidation of conductors or affect the dimension stability of conductor patterns.
In one approach to prevent the curling of flexible wiring boards, a first polyimide precursor having a relatively low coefficient of linear thermal expansion is sandwiched between layers of a second polyimide precursor having a relatively high coefficient of linear thermal expansion to make the total coefficient of linear thermal expansion equal to the coefficient of linear thermal expansion of copper (See, for example, Patent Document 1). In another approach, a plasticizing additive is added to a polyimide having a high glass transition temperature and a low coefficient of linear thermal expansion so that the polyimide is plasticized at a desired temperature (See, for example, Patent Document 2).
Furthermore, a flexible printed board has been proposed in which a polyimide resin layer is arranged in contact with a conductor. This polyimide resin layer is formed by imidization of a polyimide precursor containing a polyamic acid component and a polyimide component. The polyamic acid component is obtained by the reaction of an acid anhydride component with an amine component, and the polyimide component is obtained by the reaction of the acid anhydride component with an isocyanate component (See, for example, Patent Document 3). In still another approach, a polyimide precursor is used that has a lower coefficient of linear thermal expansion than copper foil. In this manner, the contraction of the polyimide film can be accommodated. Another technique involves dissolving a soluble polyimide in a solvent, drying the soluble polyimide and then depositing the dried polyimide on a copper foil to form a film.    [Patent Document 1]    Japanese Patent Application Laid-Open No. 2000-188445    [Patent Document 2]    Japanese Patent Application Laid-Open No. 2001-177201    [Patent Document 3]    Japanese Patent Application Laid-Open No. 2000-022288